Enhanced processing of a stimulus can occur because certain features automatically interrupt and attract processing activity or because a stimulus benefits from an already established attentional set. The proposed research assesses the control of processing provided by transient-dependent and orienting/attentional systems in the young human infant. The methods employed will examine these control systems through the modulation of heart rate, startle blink, and "prepulse inhibition" of blink. The latter is a relatively low-level effect which depends on a transient change in stimulation occurring shortly before a reflex-eliciting probe. The methods have the advantage of being useful across the life span and with otherwise inaccessible populations. Transient activity is of interest because of evidence that deficient processing of transients may characterize young infants and may underlie the perceptual difficulties observed in the aged, as well as in dyslexic and dysphasic children. By tracking the effects on prepulse inhibition of variations in the quality, intensity, and temporal properties of the prepulse, the research will determine what stimuli may be effective for the immature transient system. The possible role of transients in the facilitation of encoding will be assessed by examining whether optimal or repeated transients lead to better discrimination of stimulus change, as indexed by the dishabituation of cardiac orienting. The research also examines blink modification evidence for efferent attentional control of processing at the brainstem level. The existence of such a mechanism would have important implications for theories of attentional function and dysfunction. A receptor-adjusting explanation will be tested by 1) assessing differences in attentional modulation of early oligosynaptic and a late polysynaptic component of the cutaneous blink reflex and 2) determining whether inputs which overlap peripherally are nonetheless processed differently. A post-perceptual explanation will be tested by determining whether attention to novelty modulates blinks elicited by such stimuli or only blinks elicited by subsequent probes.